This invention relates in general to vehicle power steering and in particular to a system and method for robust active disturbance rejection in an electric power steering (EPS) assembly.
The EPS assembly for a vehicle may experience an undesirable disturbance. For example, the disturbance may be a harmonic disturbance that comes from steering wheel shake or nibble. The steering wheel shake or nibble may be due to mass imbalances on steerable wheels of the vehicle or brake disc thickness variation caused by warping or bearing/caliper alignment. Known methods of providing an active disturbance rejection capability include feedforward cancellation and attenuate system gain.
Implementing the disturbance rejection via feedforward cancellation typically involves two steps: detecting the disturbance and generating a command for an actuator to counteract an effect of the disturbance. However, accurately detecting the disturbance usually requires a certain amount of transient time. Feedforward cancellation may not be applicable in scenarios such as brake pulsations where a frequency of the disturbance is both proportional to a wheel speed and changing during braking as the wheel speed changes.
The disturbance is typically detected via different types of filters. For the EPS assembly, the filter will also filter out a same frequency component present in an original torque produced by an EPS motor. This will affect steering feel and require an iterative calibration process to balance steering feel and the active disturbance rejection capability. As a result, a detected disturbance includes not only an actual disturbance but also signals from the original motor torque. Therefore, it is not possible to reject the entire actual disturbance without also rejecting the signals from the original motor torque. The disturbance rejection can be increased by high gain, but doing so will make the feedforward cancellation more unstable and sensitive to parameter uncertainties. Finally, feedforward cancellation is computationally intensive and requires significant computational resources and memory for trigonometric calculations. Feedforward cancellation also introduces complexity in analyzing performance of an implementation in terms of stability and effectiveness because the implementation is inherently a non-linear system.
The disturbance rejection is achieved via attenuating system gain within a disturbance frequency range by lowering the system gain such that energy from the disturbance is less perceivable to a driver of the vehicle as the driver manipulates a steering wheel. Attenuating system gain is a real time strategy that will respond instantaneously to the disturbance. This makes attenuating system applicable to scenarios such as brake pulsation compensation. However, attenuating system gain also cannot reject all of the disturbance, not even theoretically. Attenuating system gain also has a significant effect on steering feel and requires retuning the EPS assembly.
The disturbance is not available for direct measurement at a rack of the EPS assembly. Instead, the disturbance is indirectly measured at a steering column where gain and phase changes due to a mechanical path from the rack to the steering column also need to be compensated for. Compensating for the gain and phase changes requires a dynamic model. Often, the dynamic model itself is not obtained accurately and causes improper gain and phase compensation.
Thus, it would be desirable to reject disturbances to the EPS assembly such that baseline performance of the EPS assembly is maintained and effects due to system parameter changes are corrected by inherent feedback mechanisms.